This invention relates to a transmitting and receiving apparatus, more particularly to a transmitting and receiving apparatus in which a single reversible electro-acoustic transducer is used for both transmitting and receiving purposes.
A typical prior art loudspeaker telephone set shown in FIG. 1 comprises a microphone 10, a loudspeaker 11, a transmitter amplifier 12, a receiver amplifier 14, a hybrid transformer 15 connected to a telephone line 17, and a voice switch circuit 16 which are connected as shown. However, in a loudspeaker telephone set constructed as shown in FIG. 1 as the hybrid transformer 15 cannot provide a perfect sidetone balance for various impedances of the telephone line, the transmitted voice current flows through a closed loop circuit including microphone 10, transmitter amplifier 12, hybrid transformer 15, receiving amplifier 14, loudspeaker 11 and microphone 10 so that a howling is created thereby making impossible the conversation.
For this reason, in the design of a prior art loudspeaker telephone set, for the purpose of decreasing the acoustic coupling between the loudspeaker and the microphone, it has been necessary to take into consideration such various factors as the directional characteristics of the loudspeaker and the microphone, the anti-howling effect between the loudspeaker and the microphone, and the acoustic characteristics of a room in which the telephone set is installed. These factors cause difficult problems especially when the loudspeaker and the microphone are contained in the same casing. In recent loudspeaker telephone sets, for the purpose of preventing howling, it has been proposed to provide a voice switch circuit 16 which functions to lower the gain of the receiver amplifier during transmission and to lower the gain of the transmitter amplifier during reception. However, although this measure can prevent the occurence of howling there is a tendency of losing the leading and trailing ends of the voice thereby greatly degrading the quality of the speech.